The present invention relates to papermaking processes and products made from these processes. More particularly, the present invention relates to papermaking processes wherein sludge generated from a papermaking process is recycled in the same process.
The manufacture of paper involves blending, in water, a pulp material (generally wood fiber) with fillers, such as clay, and other additives to create a stock slurry mixture referred to herein as a pulp. The pulp is then processed through a papermaking machine to form a sheet. The water is then extracted from the sheet and the sheet is then pressed and dried, thereby forming a paper product. The drained water contains an amount of fiber and filler material. This material is collected for later processing, however, the recovery is usually not complete. Discarded material and material not captured for reuse are generally transported to a waste treatment facility where still-remaining solids, e.g., the fibers and filler materials, are removed. The cleaned water is discharged back into the environment or communicated back to the papermaking process for reuse. After dewatering, the solids are contained in a concentrated, typically 40%–60% solids, papermaking sludge. The main components of this sludge are fibers and clay filler material. This sludge is usually disposed of by burying in landfills, landspreading, or incinerating. Some papermaking processes recycle papermaking sludge, however, this has been found to adversely affect sizing and cause size reversion.
In papermaking, “sizing” refers to the ability of paper or paperboard to resist water adsorption, water penetration, or both. “Sizing” also refers to sizing materials, sizing compositions, and sizing additives, such as an alkenyl succinic anhydride (ASA) sizing emulsion. When sizing is lost during a papermaking process, or during storage, printing, processing, or converting, the loss is referred to as a “size reversion.”
Size reversion may occur without the loss of sizing material. Extractables, hydrolyzed sizing, or both, can mask sizing in the paper or paperboard product without a loss of sizing material. In some processes, extractables in the product promote the loss of sizing. In processes where papermaking sludge is added back into the papermaking process, significant amounts of sizing materials can be lost and increased size reversion occurs.
The addition of different additives and certain processing conditions can also adversely affect sizing and increase sizing reversion. The addition of recycled sludge in a papermaking process often introduces such additives or conditions.
A need exists for a papermaking process, particularly a process wherein papermaking sludge is recycled in the process, that minimizes the loss of sizing material in the resultant paper or paperboard product, and minimizes size reversion.